Lignin from tree barks: chemical structure and valorization.

The lignins from different barks, including Norway spruce ( Picea abies ), eucalyptus ( Eucalyptus globulus ), mimosa ( Acacia dealbata ) and blackwood acacia ( A. melanoxylon ), were thoroughly characterized. The lignin from E. globulus bark was enriched in syringyl (S) units, with lower amounts of guaiacyl (G) and p -hydroxyphenyl (H) units (H:G:S of 1:26:73), which produced a lignin highly enriched in β -ether linkages (83%). The lignins from the two Acacia barks presented similar compositions (H:G:S of ~5:50:45), with a predominance of β -ethers (73-75%) and lower amounts of condensed carbon-carbon linkages; the lignin from A. dealbata bark also included some resorcinol-related compounds, not yet fully identified, that appear to be incorporated or intimately associated to the polymer. The lignin from P. abies bark was enriched in G units, with lower amounts of H units (H:G of 14:86); this composition made this lignin depleted in β - O -4' alkyl-aryl ether linkages (44%) and enriched in condensed linkages. Interestingly, this lignin presented large amounts of hydroxystilbene glucosides that are integrally incorporated into the lignin structure. This study indicates that lignins from tree barks can be seen as an interesting source to obtain valuable phenolic compounds. Moreover, this study is useful for tailoring conversion technologies for bark deconstruction and valorization.